The technical field of this invention is dielectrometry and, in particular, the electromagnetic interrogation of materials of interest to deduce their physical, chemical, geometric, or kinematic properties. The disclosed invention applies to semiconducting, both lossy and lossless dielectric media, very thin metalizations, and shape/proximity measurements for conducting and dielectric objects and surfaces.
Dielectric sensors are commonly used for material property characterization and defect detection in a material under test (MUT). The sensors respond to the absolute properties of the MUT, such as the electrical permittivity, electrical conductivity, thickness, and proximity, and changes in those properties. Factors that affect the dielectric properties include composition, chemistry and the state of cure, density, porosity, and contamination with other substances such as moisture. The property variations may be a normal part of the manufacturing process or a result of the presence of defects or damage. These defects can be created during the manufacturing process, such as improper curing or incorrect layer thickness for stratified media, or when the material is placed into service by use- and/or age-related degradation processes, such as fatigue. In manufacturing, the continuing drive toward defect-free products, yield improvement and operation near the capability limits of the production system require sensing technologies for monitoring as many critical process variables as possible. In operations, service maintenance, and repair and replacement activities, the continuing push toward a retirement-for-cause philosophy from the retire-for-time approach requires reliable measurements on all fatigue-critical components in the system, even at difficult-to-access locations.
Dielectric measurements can be performed with a wide variety of devices. The simplest devices involve parallel plate capacitors where the MUT is placed between a pair of electrodes. Often guard electrodes are used to minimize the effects of fringing electric fields at the electrode edges so that MUT is exposed to an essentially uniform electric field. The electrical terminal admittance or impedance of the device is then related to the material properties through geometric factors associated with the sensor geometry.
In many applications both sides of the MUT are not easily accessible and single-sided sensor configurations are required. A common implementation of a single-sided sensor is the interdigitated electrode structure used for chemical and moisture sensing applications. U.S. Pat. No. 4,814,690 further discloses the use of multiple sets of interdigitated electrodes as part of the imposed frequency-wavenumber dielectrometry approach for spatial profiling of stratified dielectric media. These devices have been effective in determining the dielectric properties of fluids. However, the determination of solid dielectric properties is more complicated due to the presence of microcavities or unintentional air gaps between the solid dielectric and the sensor.
While one can attempt to compensate for the air gap, the thickness is usually unknown and variable across the surface of the sensor. Therefore, effective compensation may be difficult to achieve even with multiple sensors placed onto a single substrate. One of the difficulties is due to the fact that a sensor having a number of sensor elements, each with different electrode spacings, those sensor elements are not co-located and therefore are not located at exactly the same places relative to the MUT.
Generally dielectrometry measurements require solving of an inverse problem relating the sensor response to the physical variables of interest. Such inverse parameter estimation problems generally require numerical iterations of the forward problem, which can be very time consuming often preventing material identification in real-time. Real-time parameter estimations often need to be provided for such applications as manufacturing quality control. In some cases, simple calibration procedures can be applied, but these suffer from requiring and assuming independent knowledge about the properties. More advanced model-based techniques utilize multivariable parameter estimation algorithms to estimate the properties of interest, but these are generally slow, precluding real-time measurement capabilities, and may not converge on the desired solution.
The present invention comprises of a method for generating property estimates of one or more preselected properties or dimensions of a material. Specific embodiments of the methods are disclosed for generating, calibrating, measuring properties with, and selecting among two-dimensional response databases, called measurement grids, for both single wavelength dielectrometry applications and multiple wavelength dielectrometry applications.
One step in a preferred method requires defining or estimating the range and property estimate tolerance requirements for the preselected properties or dimensions of the material under test. The next step is the selecting at least one of each of an electrode geometry, configuration, excitation source, and measurement instrument operating point. A continuum model, either analytical or numerical or an experimental approach using calibration test pieces of known properties and dimensions or both are then used to generate measurement grids as well as operating point response curves for preselected operating point parameters.
The measurement grids and operating point response curves are subsequently analyzed to define a measurement strategy. Operating point parameters and an electrode geometry, configuration, and excitation source are then determined to meet the dynamic range and tolerance requirements. To accomplish this, property estimation grids and operating point response curves are generated and analyzed for various operating points. The sensitivity and selectivity is calculated for grids representing varying electrode designs and operating conditions. Then the best of the lot of prechosen design parameters and operating conditions are selected. If inadequate to requirements, this evaluation process can be reiterated with improved selections based on what was learned in prior rounds.
A property estimator implements a model for generating a property estimation grid, which translates sensed responses into preselected material property or dimension estimates. Accordingly, the present invention includes a method for generating a property estimation grid for use with a dielectrometer for estimating preselected properties or dimensions of a material under test. The first step in generating a grid is defining physical and geometrical properties of the MUT and the electrode geometry, configuration, and source excitation for the dielectrometer are defined.
The material properties, the operating point parameters, and the dielectrometer electrode geometry, configuration, and source excitation are input into a model to compute an input/output terminal relation value. In a preferred embodiment, the input/output terminal relation is a value of transadmittance magnitude and phase. The terminal relation value is then recorded and the process is repeated after incrementing the preselected properties of the material under test. After a number of iterations, the terminal relation values are saved in a database of material responses and plotted to form a property estimation grid.
A preferred embodiment of the method according to the invention includes the incorporation of geometric properties into the grid databases for the representation of multi-layered media and the use of analytic properties of the measurement grid to map from measurement space to property space, such as singular value decomposition, condition numbers (and visualizations of these), to improve selection amongst alternative sensor designs and operating conditions. In addition, the preferred embodiment uses methods for measuring with and calibrating dielectrometers, using single and multiple grids for multiple wavenumber dielectrometry. Exploiting the characterization and understanding of other properties of such mappings to aid in choosing and selecting among measurement grid alternatives is also feasible.
A preferred method of the invention includes enhancing sensitivity and selectivity by using fluids or solids of known properties and dimensions to intentionally move the sensor response within the grid or to alter the grid itself. Movement within grids can also be achieved by varying other parameters such as temperature that also affect permittivity. The method supports both measurement and calibration. One method uses xe2x80x9cshimsxe2x80x9d of known dielectric constant, conductivity, and thickness between the sensor and the material under test.
The need is also recognized for a sensor device configuration that reduce the sensor sensitivity to undesired inhomogenities across the face of the sensor. In a preferred embodiment, a sensor has multiple electric field penetration depths but each with the same air-gap, fluid gap, or shim lift-off height, thereby greatly reducing the number of unknowns in parameter estimation algorithms.